Genome evolution in a conserved yet diverse symbiotic community

Genomes of insect symbionts are subjected to severe reductions in size compared to their free-living ancestors, resulting in “streamlined” genomes retaining only the genes beneficial to the symbiosis. However, most examples of this pattern are found in intracellular, non-diverse habitats such as in aphids and their symbiont Buchnera. In contrast to other well-known stories of reductive genome evolution in conserved symbioses, the gut symbionts of turtle ants (Cephalotes spp.) exist in a diverse community spanning nine orders. Genomes from the core Cephalotes symbionts have previously been sequenced, providing information about how the microbes may be providing their ant host with valuable services including nitrogen recycling and amino acid provisioning. Currently lacking, however, is an understanding of how the genomes of these symbionts have been shaped by their restriction to the host gut. Taking an ancestral state reconstruction approach aided by phylogenomic trees for each symbiont group, we show that the genomes of these symbionts have universally reduced in size since entering the symbiosis, although mildly so. These patterns reflect large-scale gene loss, with some genes convergently lost among unrelated symbiont taxa. On the other hand, reconstruction results also suggest that several genes have been inserted into their genomes since entering the symbiosis. Phylogenetic analyses suggest that new genes may have been horizontally acquired from other symbionts in the community, perhaps providing an adaptive benefit to the recipient. Ongoing work will focus on how these patterns have impacted interactions within the symbiont community, as well as with their ant host.